Version one was tied to a "5 knot" marker buoy with some string:

This was effective, but not ideal. There's no way to remove the sensor without undoing everything. It's hard to tie knots when you're bobbing around in a wetsuit in the water. And it just doesn't look good.

So time to find something better!

How big does it need to be?

I've never swum out with the ruler to measure how big the marker is, but i figure they must be a "standard" size, right? So much Googling later, yeah there's not really a standard size for these in NZ. But they they look almost exactly like the "Marker Buoy 400" from an NZ company. They website has a handy drawing:
I "verified" a couple of measurements by using hand spans while out swimming one day and it all matched up. The below the waterline section is different but that doesn't affect me.

What to make it out of?

I need it to be kinda stealth (still haven't work out if I'm allowed to attach to the marker buoys) so needs to be yellow. I tried to buy some yellow webbing from my normal sources, 100m rolls was the shortest I could order as it's not a standard color. I don't need 100m :-(

I was also going to need a buckle of some sort, what has 25mm webbing and a solid buckle? Kayak tie downs, and they even come in yellow! Someone had bought some Multi-purpose Tie down Straps into NZ so I bought four for less then I could have paid for the webbing alone. I also got some polyester thread for my sewing machine so it didn't rot like cotton might.

Putting it all together

Maintaining the same basic idea as the tied version. A loop at the top, a loop around the middle with the buckle in it, and a vertical length between them. Then some loops and velcro to hold the enclosure firmly but also be removable. 30mins later it was all together and ready to take out and try.

The top fitted great

The bottom was a bit high, next version the vertical piece will be longer so the lower loop doesn't cover the words on the buoy. Oh and I printed and laminated some labels and threaded them on the lower loop. The black velcro secures the enclosure very firmly.

Sea trial two began Sunday 14th June with two units installed, each on different beaches. Both are sending data to http://oceanswims.nz/watertemp